For tail fins, I put a dab of epoxy on the inside portion of a plastic milk carton. I then add some fin material (I use DNA Fish Fiber from Cabelas). I then sandwich the fibers between another piece of milk carton, leaving the fin material sticking out. Make sure the epoxy penetrates through the thickness of the fin material. I can then carefully shape the fin material to my liking, and leave it to dry. I use 5 minute epoxy and allow it to sit for 30-60 minutes.
The epoxy will not stick to the carton material. After it is dry simply peal it off, and you have a fin. Both ends of the fin can be trimmed as needed to properly fit your bait. Hope this makes sense.
Thanks Sammy, now one more thing to buy for the lure building obsession. LOL I will give this a try on the next one.